A prominent Republican senator unloaded on Donald Trump in a testy interview

One of the last Republicans in the Senate to hold out on endorsing Donald Trump expressed major problems with his candidacy Wednesday.

In an interview with Newsmax, host JD Hayworth pressed Sen. Mike Lee to endorse Trump, citing presumptive Democratic presidential nominee Hillary Clinton's coziness with foreign leaders and use of a private email server while serving as secretary of state.

The Utah senator balked at the suggestion, pointing out his concerns with Trump's actions on the campaign trail, including his proposed ban on Muslims entering the US and his unfounded claim that Sen. Ted Cruz's father was involved in President John F. Kennedy's assassination.

"I get it. You want me to endorse Trump," Lee said.

"We can get into that if you want. We can get into the fact that he accused my best friend's father of conspiring to kill JFK. We can go through the fact that he's made statements that some have identified correctly as religiously intolerant. We can get into the fact that he's wildly unpopular in my state, in part because my state consists of people who are members of a religious minority church. A people who were ordered exterminated by the governor of Missouri in 1838. And, statements like that make them nervous."

While Lee acknowledged that he could still endorse Trump if he changes his campaign, the senator said he's still worried about Trump's character.

"If you want to why it is I have concerns, I can go on if you like," Lee said. "But don't sit here and tell me that I shouldn't have concerns about Donald Trump."

While many national Republicans have urged senators to coalesce behind Trump, there's little pressure at home to for the former Cruz backer to endorse Trump.

Trump remains notably unpopular in Utah. Despite its status as a deep-red conservative state, a recent state poll showed the presumptive Republican presidential nominee virtually tied with Clinton, with Libertarian presidential nominee Gary Johnson garnering 13% support. Trump finished third in the Republican primary in Utah earlier this year, winning just 14% support to Cruz's 69% support.

Privately, some Clinton campaign staffers believe Utah is a state where the former secretary of state could perform surprisingly well, though the campaign is still deciding whether to seriously invest in attempting to flip the reliably conservative state.